In the experiment, the microwave radiation at a tunable frequency in the range \(\omega /2\pi =400\) -500 GHz is introduced into the cell containing the microchannel device from a room-temperature source via a MW guiding system. The main part of the guiding system is an overmoded (WR-28) stainless-steel waveguide, while its short portion entering the cell is an overmoded (WR-06) copper waveguide sealed with a piece of 50-\(\mu \) m thick Kapton film. The input MW power could be adjusted using an attenuator at the output of the MW source. At a given attenuator setting, the power output of our MW source strongly depends on the MW frequency. Therefore, in the experiment we kept the frequency of MW radiation fixed and tuned the Rydberg transition frequency \(\omega _{21}\)  of SE in the central channel via the Stark effect by adjusting the dc (pressing) electric field \(E_\perp \)  acting on SE perpendicular to the surface of liquid [1]}. Practically, this was done by varying the voltage \(V_\textrm {ch}\)  applied to the bottom electrode of the central channel. We note that simultaneously it caused variation of the electron density \(n_s\)  in the central channel, which increases with increasing \(V_\textrm {ch}\) .
